Traffic signal priority (TSP) is an operational strategy that facilitates the movement of in-service transit vehicles through traffic-signal-controlled intersections. The TSP modifies the normal signal operation process to accommodate the transit vehicles to reduce the delay time of a transit vehicle at an intersection. The TSP also attempts to minimize impact on other vehicles crossing the same intersection from different directions. The traffic signal priority can improve schedule adherence, transit efficiency, and accuracy of the transit information as well as increase overall road network efficiency.
Note that traffic signal priority is different from traffic signal preemption because traffic signal preemption interrupts the normal process for special reasons (e.g., as train approaching, emergency vehicle or police car responding to an emergency call, etc.), while traffic signal priority modifies the normal signal operation process rather than interrupting it. There are generally two types of approaches to providing traffic signal priority. The basic form is local intersection TSP, which is accomplished at a local intersection level by using a first detector that detects a vehicle approaching the intersection and sending a “check-in” call to a traffic signal controller. A second detector detects the vehicle as it passes through the intersection and sends a “check-out” call to the traffic signal controller to notify the controller of this fact.
This system has several drawbacks, however. First, it requires installation of a detector and/or receiver at each TSP intersection to detect vehicles approaching the intersection. It also lacks the ability to generate detailed information related to the vehicle's speed, which relates to the vehicle's estimated time of arrival (ETA) to the intersection. These limitations may affect the accuracy of triggering the TSP request. Moreover, it is difficult to incorporate any traffic control strategy algorithm into this TSP approach because this approach does not provide the information on the many factors needed for the algorithm to calculate strategies for improving the traffic flow, minimizing the impact of traffic flow from other directions, and ensuring the safety of crossing pedestrians.
A more sophisticated approach incorporates an automated vehicle location (AVL) and control (AVLC) system that communicates with either a traffic signal controller at the intersection and/or a centralized control center in a network. The control center sends a priority request to a traffic signal at the intersection through a network connection. One common implementation of this approach requires the vehicle to transmit GPS position data periodically to the control center. The control center then calculates the speed and estimated arrival time in combination with other information to determine if TSP is needed at the intersection. If so, the control center will send a corresponding TSP request or cancellation to the traffic signal controller.
If the TSP request or cancellation is determined at the control center in a network-based TSP system, the accuracy of issuing the request is based on the frequency of the mobile unit (e.g., the vehicle) transmitting the mobile location messages to the control center. A higher frequency of reporting vehicle locations may tend to overload the radio or other components in the system, while a lower frequency of reporting may cause inaccurate timing in triggering a change in the traffic signal. The transfer rate normally is on the order of one message per second (1 Hz). However, heavy data transfer between mobile and the control center can cause radio congestion, especially for large cities with many transit vehicles operating at same time and having a high demand for TSP.
There is a desire for a method and system that can conduct traffic signal control more accurately to optimize traffic flow.